1. Field of the Invention
The present invention relates to viscous fluid coupling devices adapted for use in automotive vehicles, and more particularly to a viscous fluid coupling device capable of controlling the transmission of an output torque in response to the ambient temperature in three steps.
2. Discussion of the Background
A conventional viscous fluid coupling device of this kind is adapted to control the rotational speed of the radiator cooling fan mounted on an output member of the device. The conventional viscous fluid coupling device comprises a housing rotatably mounted on an input shaft to store therein a predetermined amount of viscous fluid, a rotor fixed to the input shaft and located in the housing, and a partition plate secured within the housing to subdivide the interior of the housing into an operation chamber containing therein the rotor and a portion of the viscous fluid and a reservoir chamber storing therein the remaining portion of the viscous fluid, the partition plate being provided at its outer peripheral portion with a first slot and a second slot spaced radially inwardly from the first slot. The fluid coupling device further comprises a first labyrinth between the partition plate and the rotor, a second labyrinth between the rotor and the inner wall of the housing, a valve plate rotatable in the reservoir chamber to open and close the slots to control the quantity of viscous fluid supplied into the labyrinths from the reservoir chamber respectively through the slots, thermally responsive means arranged to rotate the valve plate in response to changes in the ambient temperature of the coupling device, and pump means for returning the viscous fluid into the reservoir chamber from the operation chamber in accordance with relative rotation between the rotor and the housing.
In operation of the viscous fluid coupling device, the following first, second and third operational modes are effected in response to a rise in the ambient temperature of the coupling device. In the first operational mode, both the first and second slots are closed by the valve plate to minimize the shearing force of the viscous fluid in the first and second labyrinths so as to maintain the transmission of an output torque at a low level as indicated by the character B in FIG. 1. In the second operational mode, only the first slot is opened to increase the shearing force of the viscous fluid in the first labyrinth so as to maintain the transmission of the output torque at a medium level as indicated by the character C in FIG. 1. In the third operational mode, both the first and second slots are opened to further increase the shearing force of the viscous fluid in both the labyrinths so as to maximize the transmission of the output torque as indicated by the character D in FIG. 1. However, in the case that the fluid coupling device is supplied with high viscous fluid or designed to reduce relative rotation between the rotor and the housing as small as possible, the third operational mode changes to the first operational mode in response to fall of the ambient temperature without effecting the second operational mode as is indicated by the character A in FIG. 1. Such phenomenon is caused by the high viscous fluid remained in the second labyrinth after closing of the second slot or by insufficient pumping operation in the third operational mode. This results in excessive cooling of a prime mover of the vehicle, increase in noises of the cooling fan, increase in the power loss and the like.